| Date | November 2013 | Marks available | 3 | Reference code | 13N.3.sl.TZ0.20 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Explain | Question number | 20 | Adapted from | N/A |
Question
Stearic acid, oleic acid and linolenic acid are all fatty acids that contain 18 carbon atoms. Their structures are given in Table 22 of the Data Booklet.
Partial hydrogenation of linolenic acid may lead to a product known as a trans fatty acid.
Explain which acid has the highest melting point.
Discuss two potential problems or health concerns associated with trans fatty acids.
Markscheme
stearic acid;
saturated molecule / more closely packed / greater surface area (of contact) / not “kinked”;
more/stronger van der Waals’ forces;
Accept intermolecular/London/dispersion forces instead of van der Waals’ forces.
trans fats harder to metabolize / accumulate in tissue;
increase levels of LDL cholesterol/low-density lipoprotein / increase risk of heart disease;
low-quality energy source;
Examiners report
Even though the correct acid was often not identified, explanations for the highest melting point often gained full credit in (a). Disappointingly few candidates were able to write a correct equation in (b) (with the addition of three moles of \({{\text{H}}_{\text{2}}}\) (g)) although the conditions were usually correct. In (c), many were able to show a trans orientation – but didn’t use a fatty acid – and many did not score both marks in (ii).
Even though the correct acid was often not identified, explanations for the highest melting point often gained full credit in (a). Disappointingly few candidates were able to write a correct equation in (b) (with the addition of three moles of \({{\text{H}}_{\text{2}}}\) (g)) although the conditions were usually correct. In (c), many were able to show a trans orientation – but didn’t use a fatty acid – and many did not score both marks in (ii).
